1. Field of the Invention
The present invention relates to a method for controlling a linear expansion valve so as to suitably control the flow rate of refrigerant circulating in an air conditioning system, and more particularly to a method for controlling the aperture ratio of a linear expansion valve of a heat-pump type air conditioning system using a plurality of compressors.
2. Description of the Related Art
A general air conditioning system includes a compressor, a condenser, a linear expansion valve, and an evaporator. The compressor converts a low temperature and low pressure gaseous refrigerant into a high temperature and high pressure gaseous refrigerant. The condenser converts the high temperature and high pressure gaseous refrigerant, converted by the compressor, into a mid-temperature and high pressure liquid refrigerant. The linear expansion valve converts the mid-temperature and high pressure liquid refrigerant, converted by the condenser, into a low temperature and low pressure liquid refrigerant. The evaporator converts the low temperature and low pressure liquid refrigerant into a gaseous refrigerant.
On the other hand, the heat-pump type air conditioning system is provided with a four-way valve which changes the flow-path size of the refrigerant according to heating and cooling modes, whereby the functions of indoor and outdoor heat exchangers are changed. Namely, the indoor and outdoor exchangers serve as a condenser and an evaporator, respectively, in the heating mode, whereas both exchangers serve as an evaporator and a condenser, respectively, in the cooling mode.
In recent times, a plurality of compressors having different capacities are used to change the compression capacity according to a heating or cooling load, so as to optimize cooling and heating efficiencies.
As shown in FIG. 1, a heat-pump type air conditioning system using a plurality of compressors according to the prior art includes a plurality of compressors 11 and 12, an outdoor heat exchanger 14, an outdoor fan 14a, an linear expansion valve 15, an indoor heat exchanger 16, an indoor fan 16a, and a four-way valve 13. The compressors 11 and 12 compress refrigerant. The outdoor heat exchanger 14 allows heat exchange between the compressed refrigerant and outdoor air to condense the compresses refrigerant into a mid-temperature and high pressure liquid refrigerant. The outdoor fan 14a ventilates outdoor air toward the outdoor heat exchanger 14. The linear expansion valve 15 decompresses the mid-temperature and high pressure refrigerant into a low temperature and low pressure refrigerant. The indoor heat exchanger 16 allows heat exchange of the refrigerant passing through the linear expansion valve 15 with indoor air to evaporate the refrigerant and refrigerate the indoor air. The indoor fan 16a ventilates the indoor air toward the indoor air. The four-way valve 13 changes the flow-path size of the refrigerant according to the cooling and heating modes.
Based on the difference between a desired heating/cooling preset temperature and actual indoor temperature T4, a controller 20 judges whether the cooling and heating loads are large or small and controls the operation of the compressors 11 and 12 so that either all or a part of the compressors are activated.
In addition, the controller 20 controls the four-way valve 13 to change the flow-path size of the refrigerant according to the cooling/heating mode, and detects pipe temperatures T1 and T2 of the indoor heat exchanger 16 and the outdoor heat exchanger 14 to control the operation of the indoor fan 16a and the outdoor fan 14a. 
Further, the controller 20 allows a suitable amount of refrigerant to be circuited according to the cooling and heating loads. The controller 20 also detects the pipe temperatures T1, T2 and the inlet-side pipe temperature T3, and controls the aperture ratio of the linear expansion valve 15 so as to maintain a suitable superheat.
However, the heat-pump type air conditioning system of the prior art has the following problems. If a connection pipe of the outdoor and indoor heat exchanger 14 and 16 is composed of a long pipe, a pressure drop occurs while the refrigerant passes through the long pipe, reducing the inlet-side pipe temperature T3. This causes the current superheat to be significantly lower than the target superheat set at the time of designing the system. Accordingly, the controller 20 judges there to be a large amount of circulating refrigerant and thus reduces the aperture ratio of the linear expansion valve 15 so as to reduce the flow rate of refrigerant. Thereby, the flow rate of circulating refrigerant is sharply reduced, resulting in a shortage of the flow rate of refrigerant compared to the cooling load.
In addition, when the connection pipe of the indoor and outdoor heat is composed of a short pipe, or the flow rate of the circulating refrigerant is insufficient, the inlet pipe temperature T3 becomes high, causing the current superheat to exceed the target superheat. This causes the controller 20 to control the linear expansion valve 15 to have a higher aperture ratio, thereby sharply increasing the flow rate of circulating refrigerant. This causes liquid refrigerant to flow into the compressors 11 and 12.
Further, the controller 20 detects indoor temperature T4 and outdoor temperature T5, and controls the operation of the indoor and outdoor fans 16a and 14a and the aperture ratio of the linear expansion valve according to the cooling and heating loads. However, if the cooling and heating loads increase, the aperture ratio of the linear expansion valve 15 sharply increases, and thus the amount of the circulating refrigerant sharply increases, consequently causing a problem in that liquid refrigerant flows into the compressors 11 and 12.
Moreover, when the air volume of the indoor fan 16a decreases, or when the outdoor fan 14a is turned off in order to prevent heating overload under a condition of high outdoor temperature during the heating operation, refrigerant passing through the indoor heat exchanger 16 or the outdoor heat exchanger 14 is insufficiently evaporated, causing liquid refrigerant to flow into the compressors 11 and 12.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for controlling a linear expansion valve of an air conditioning system, wherein the flow rate of circulating refrigerant is prevented from being deficient or excessive at any operation environment of the air conditioning system so that the refrigerant is always circulated at a suitable flow rate, thereby improving the air conditioning performance and also securing high reliability of the air conditioning system.
It is another object of the present invention to provide a method for controlling a linear expansion valve of an air conditioning system, wherein the aperture ratio of the linear expansion valve is controlled so that a suitable amount of refrigerant is circulated, irrespective of whether a connection pipe of indoor and outdoor heat exchangers is composed of a long pipe or a short pipe.
It is still another object of the present invention to provide a method for controlling a linear expansion valve in an air conditioning system using a plurality of compressors, wherein the aperture ratio of the linear expansion valve is controlled so that the liquid refrigerant is prevented from flowing into the compressors when the compression capacity of the refrigerant is changed according to cooling and heating loads.
It is yet another object of the present invention to provide a method for controlling a linear expansion valve of an air conditioning system, wherein the opening valve of the linear expansion valve is controlled according to the operation status of an indoor or outdoor fan so as to prevent the liquid refrigerant from flowing into the compressor.
In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a method for controlling a linear expansion valve of an air conditioning system, wherein a refrigerant compression capacity of a compressor is changed according to cooling and heating loads, and an aperture ratio of the linear expansion valve, mounted between an indoor heat exchanger and an outdoor heat exchanger, is controlled in response to the change of the refrigerant compression capacity, the method comprising:
a first step of determining a minimum value and a maximum value of a range within which the aperture ratio of the linear expansion valve is controlled, according to each of refrigerant compression capacities to be changed; and
a second step of, when the refrigerant compression capacity is changed according to the cooling and heating loads, controlling the aperture ratio of the linear expansion valve in the range from the minimum value to the maximum value, which are determined according to each of the changed refrigerant compression capacities.
In accordance with another aspect of the present invention, there is provided a method for controlling a linear expansion valve of an air conditioning system, wherein as a refrigerant compression capacity of a compressor is reduced from a first set value to a second set value, an aperture ratio of the linear expansion valve is reduced by a predetermined value from a first specific value to a second specific value, and
wherein when the refrigerant compression capacity of the compressor increases from the second set value to the first set value, the aperture ratio of the linear expansion valve increases from the second specific value by a value less than the predetermined value, and, after the air conditioning system operates for a predetermined time, the aperture ratio increases to the first specific value, allowing the operation of the air conditioning system.
In accordance with still another aspect of the present invention, there is provided a method for controlling a linear expansion valve of an air conditioning system wherein, while the system operates in a cooling mode, when air volume of an indoor pan decreases, the aperture ratio of the linear expansion valve decreases.
In accordance with yet another aspect of the present invention, there is provided a method for controlling a linear expansion valve of an air conditioning system, wherein when the system operates in a heating mode, if an outdoor fan is turned off, stopping its operation, an aperture ratio of the linear expansion valve is controlled to be a fourth set value which is less than a third set value by a predetermined value, said third set value being equal to an aperture value when the outdoor fan operates, and thereafter when the outdoor fan is turned on, the aperture ratio increases by a value less than the predetermined value, and, after a predetermined time, the aperture ratio increases to be the third set value.